Generally, a hybrid electric vehicle is driven by effectively combining at least two different power sources. The hybrid electric vehicle generally uses an engine driven by fossil fuel combustion and a motor driven by a rechargeable battery.
The hybrid electric vehicle can be classified as two types depending on a power source of the engine and the motor. A parallel hybrid electric vehicle is directly driven by mechanical power of the engine and uses electric power of the motor in case of need. A series hybrid electric vehicle is driven by the electric power of the motor which is converted from the mechanical power of the engine by a generator. Therefore, the parallel hybrid electric vehicle is suitable for high-speed driving or long distance driving, whereas the series hybrid electric vehicle is suitable for city driving or short distance driving.
Recently, a plug-in hybrid electric vehicle (PHEV) which is driven in an electric vehicle (EV) mode for a short distance by a larger battery than that of the hybrid electric vehicle and is driven in a hybrid electric vehicle (HEV) mode for long distance has been developed. That is, the PHEV simultaneously has an internal combustion engine driven by gasoline and a battery powered engine, so that the PHEV uses one of the two engines or both engines. The PHEV has a high capacity and high voltage battery, so that it needs to be charged by electricity from the outside.
The PHEV is driven in the HEV mode when a driver's demand for power is greater than a maximum output power of the motor, and the battery or a battery state of charge (SOC) is low. On the other hand, the PHEV is driven in the EV mode when the driver's demand for power is lower than the maximum output power of the motor, and the battery and the battery is charged by the electricity from the outside.
However, if the PHEV is driven in the EV mode without operating the engine for a relatively long period of time to improve fuel efficiency, corrosion, leakage of a lubricant, fuel damage, and so on can occur. Therefore, the plug-in hybrid electric vehicle should drive the engine periodically even though the engine operation is unnecessary for driving.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.